The intermediate circuit voltages in large-scale power converters have already reached approximately 5 kV today and tend toward 10 kV and 20 kV. Such high intermediate circuit voltages also make high demands of the insulation resistance of the current and voltage transformers. The partial discharge strength is the most important design parameter for the insulation resistance. At an intermediate circuit voltage of 5 kV, a partial discharge strength of 20 kV must be required.
The transformer systems in use today are compensating current transformers. To maintain the required partial discharge strength, however, these transformers must be manufactured in accordance with techniques that greatly increase the cost of the transformers. Furthermore, these transformers take up a great deal of space, and the lines that provide power to the transformer and that transmit measurement signals must be installed in compliance with specifications for the clearance in air and the creepage distance to guarantee reliable isolation. This further increases the amount of space required.
Since transformers can hardly guarantee a partial discharge strength of 20 kV at an intermediate circuit voltage of 5 kV, increasingly expensive insulation is not a suitable way to make, at a relatively inexpensive cost, a strong transformer system suitable for even higher intermediate circuit voltages.
A fundamentally different approach toward solving this problem is to achieve reliable isolation by means of light guides. However, this means that an electronic component connected to the sensor is needed to amplify at least the measurement signals supplied by the sensor and to control a light guide transmitter. However, the power supply to this electronic component must then also be transmitted over a light guide. Such power transmission systems comprise a laser, a light guide, and a power converter. However, the transmissible power is limited to a few hundred milliwatts. For this reason, no compensating current transformer can be used in systems that are supplied with light energy because the power demand by these transformers is too great due to the compensating current.
The article "Optical Current Transformer--Successful First Field Trial In A 380 kV Network", printed in the German journal ABB Technik, vol. 3 (1994) pages 12 through 18, discusses an active optical current measuring system. This article presents various optical current transformers that have been tested in a German 380 kV network under actual conditions. The active current transformer functions essentially according to the conventional current measurement principle, supplemented by a digital optical transmission link. Specifically, the system comprises an air-core inductor, including load impedance, an analog-digital converter, and a transmitter unit with a light-emitting diode at a high potential. A light guide establishes the connection to the interface device at ground potential. The electronic component that is at a high voltage potential has a power demand of less than 150 .mu.W. This electronic component is designed as a low-power, CMOS integrated component. A laser diode that transmits power between two data telegrams over the same light guide is sufficient as the power supply. At greater distances between the sensor and the interface device, a parallel light guide is needed for the power supply because of the higher transmitting power.
The article "EHV Series Capacitor Banks. A New Approach To Platform To Ground Signaling, Relay Protection And Supervision," published in the journal IEEE Transactions on Power Delivery, vol. 4, no. 2 (April 1989) pages 1369 through 1378, describes the use of an active current transformer with a series compensator. This article describes the design of an active current transformer, an electronic sensor component and an interface device at ground potential. The electronic sensor component comprises a filter, a plurality of voltage dividers, and an energy converter. The interface device comprises a receiver, a limit value circuit, a detector, a buffer, a pulse generator, a transmitter, and a light filter. Since the active current transformer uses an air-core inductor with a load impedance for the current measurement, only alternating currents can be detected with this active current transformer.
A measuring resistor or shunt is a current sensor that can also detect direct currents and does not need a separate power supply. Since the measurement signals of the shunt must be kept low in the interest of keeping the power loss low, an electronic sensor should be located in the immediate vicinity of the shunt to prevent most interference. This electronic component is then operated in a rough electromagnetic environment, i.e., it is exposed to strong and often quickly changing electric and magnetic fields. In addition, the electronic component is usually exposed to relatively high temperatures. One disadvantage of the measuring resistor is that it must be machined for an adjustment.